The signals are blaring: Dramatic changes to our climate are well upon us. These changes — we know thanks to a steady drumbeat of alarming official reports over the past 12 months — could cripple the U.S. economy, threaten to make vast stretches of our coastlines uninhabitable, make basic food supplies scarce and push millions of the planet’s poorest people into cities and across borders as they flee environmental perils.

Global emissions of carbon dioxide have reached the highest levels on record, scientists projected Wednesday, in the latest evidence of the chasm between international goals for combating climate change and what countries are actually doing.